The yarn tension target profile for the insertion cycles has to be adjusted during the first setting-up or after a changeover of the yarn processing system to another yarn quality (style change). A yarn tension target profile is selected which guarantees optimal insertion frequency and insertion speed with a minimum number of yarn breakages for the respective yarn quality. The yarn tension is influenced by a plurality of parameters, e.g. the withdrawal tension from the yarn feeding device, the braking effect of the yarn brake, the type and function of the insertion device of the textile machine, the yarn quality, and the like.
Even characteristics of the yarn like the rubbing property, the diameter, the elasticity, or the density are decisive for the resulting yarn tension profile. Those parameters need certain adjustments e.g. at braking devices influencing the yarn tension. Deviations from the set yarn tension profile needing compensation may even sometimes occur during operation of the yarn processing system, e.g. caused by different diameters of the supply bobbins, fluctuating yarn characteristics and differently spooled supply bobbins. The textile machine ought to process the yarn as quickly as possible for obvious reasons. In case of weak yarns the strength of the yarn sets a limit. If then the machine speed is raised beyond a critical limit the number of yarn breakages increases exponentially. The highest tension peaks caused by the high insertion speed may be reduced by means of a controlled yarn brake such that the tension remains close to lower values during particularly critical phases of the insertion. For this purpose high grade controllable and adjustable yarn brakes already exist. The precise setting of those yarn brakes is complicated such that they gained only limited positive influences on the processing efficiency in the yarn processing system in practice. Controlled yarn brakes can be adjusted optimally only with the information of the actual tension or the actual tension profile, respectively, during an insertion cycle. The information on the yarn tension can be obtained with the help of the tensiometer. The tensiometer, however, means an additional yarn friction angle during the measurement of the yarn tension. This additional friction angle, caused by the tensiometer, may mean a catastrophe for weak yarns, because the additional tension generated by the tensiometer increases the likelihood of yarn breakages dramatically such that the tensiometer cannot be used for a continuous operation of the yarn processing system with weak yarns. In the setting phase and until an optimal yarn tension target profile is adjusted this disadvantageous influence of the tensiometer on weak yarns, however, can be tolerated. Stronger yarns, which are processed with the help of a controlled yarn brake, to the contrary, can stand the detrimental influence of the tensiometer without an increase of the likelihood of yarn breakages even during constant operation.
It is known to employ a portable tensiometer which is put in the yarn path, measures the yarn tension, and, in some cases, shows the yarn tension on a laptop. The tensiometer is used for a number of insertion cycles which is representative of the adjustment of the yarn tension target profile, in order to adjust e.g. the withdrawal tension at the yarn feeding device, the braking level or timing of the yarn brake, and the like. During this adjustment phase yarn breakages or other disturbances may occur to a certain extent, until finally the optimal yarn tension target profile is found and established.
A yarn processing system known from EP 0 357 975A (corresponding to U.S. Pat. No. 5,050,648) employs a controlled yarn brake which is operated with the help of a tensiometer which is placed permanently in the yarn path. The tensiometer, permanently operating in its detection position would allow to adjust an optimal yarn tension target profile, however, the influence of the tensiometer is a drawback for weak yarn qualities because of the additional yarn deflection and yarn friction.
EP 0 605 550 A (corresponding to U.S. Pat. No. 5,462,094) discloses a yarn processing system having a tensiometer which is permanently associated to a controlled yarn brake and which is adjustable between a passive position and a detection position. Since the tensiometer is adjusted to the detection position only temporarily during each insertion cycle, namely when simultaneously the yarn brake is operating, no information on the yarn tension is available when the yarn brake does not brake. So to speak, the tensiometer only is able to measure a restricted section of the yarn tension profile during an insertion cycle. For an adjustment of an optimal yarn tension target profile, however, both the development and the course of the yarn tension during the entire insertion cycle are needed.
It is an object of the invention to provide a yarn processing system as disclosed at the beginning which allows high insertion speeds for different yarn qualities and with a minimal yarn breakage quota only.
Said object can be achieved by a yarn processing system comprising at least one yarn feeding device associated to a yarn channel, a textile machine like a weaving machine or a knitting machine, a yarn brake in the yarn path between the yarn feeding device and the textile machine, which yarn brake at least is adjustable, and a tensiometer at least for measuring the yarn tension, which tensiometer scans the yarn downstream of the yarn brake, which tensiometer is provided permanently in the yarn path and is switchable between a passive position and at least one detection position, and which tensiometer can be selectively switched from the respective detection position into the passive position after a number of insertion cycles has occurred which number is representative at least for the adjustment of a yarn tension target profile.
As soon as the tensiometer is switched over to the detection position, the tensiometer monitors the development and the course of the yarn tension during the entire insertion cycle. The tensiometer remains in the detection position for a representative number of insertion cycles, typically for about 50 to 100 insertion cycles, until the optimal yarn tension target profile is adjusted by varying the parameters influencing the yarn tension. The optimal yarn tension target profile is a target profile which assures a minimum number of yarn breakages in case of optimal high insertion speeds. In case of strong yarn qualities the tensiometer may remain in the detection position after the adjustment, in order further on to provide permanent information on the yarn tension, because strong yarn qualities can stand the additional friction caused by the tensiometer. However, as the tensiometer selectively can be readjusted to the passive position, an optimal yarn tension target profile can be adjusted even for weak yarn qualities, in some cases first with disturbances caused by the tensiometer. The finally found yarn tension target profile guarantees a minimal yarn breakage quota for an optimal high insertion speed, however, and after the tensiometer has been switched back to the passive position. The short period of time during which the weak yarn has to stand the additional friction does not mean a significant reduction of the efficiency of the textile machine. In the case that for strong yarn qualities the tensiometer is maintained in the detection position, even during constant operation new adjustments of the parameters may be carried out, e.g. at the controlled yarn brake, when e.g. the quota of yarn breakages should have increased as a consequence of the above-described influences. In case of a non-controlled yarn brake the tension measured by the tensiometer in the detection position may be used with the help of graphical or numerical displays to manually adapt the braking level of the yarn brake.
The switchable tensiometer, expediently, is associated to a yarn brake operating with an adjustable braking level which remains unchanged during the insertion cycle, in order to vary the braking level until an optimal yarn tension target profile could be found, or is associated to a controlled yarn brake, respectively, which allows to vary the braking effect during one and the same insertion cycle. The timing and/or the braking level of the controlled yarn brake can be adjusted with the help of the information from the tensiometer.
The tensiometer, advantageously, is directly connected to an adjustment device of the yarn brake such that it may operate in a closed regulation loop with feedback. In this case a computerised control device or braking level adjustment device of the yarn brake are expedient which responds to the measured yarn tension in some cases in a correcting fashion.
In a simple embodiment the tensiometer or at least the tensiometer element which actuates the yarn during a measurement can be switched manually or mechanically. A manual switching operation may be carried out by directly engaging at the tensiometer or the element respectively. A mechanical switch over e.g. can be carried out with the help of a spring which automatically adjusts the tensiometer into the passive position after the representative number of insertion cycles has passed.
The tensiometer or the element engaging on the yarn, respectively, expediently is connected with a switch over actuator, preferably an electromagnet or an electric motor which receives the command, e.g. from a timer or a program, to adjust the passive position of the tensiometer after the representative number of insertion cycles had passed.
The handling is very comfortable if the tensiometer is provided with a display device for the measurement results, preferably a display device with a graphical or numerical indication.
Since the tensiometer is permanently provided in the yarn path it is expediently connected to the operation panel of the textile machine such that the tensiometer cannot only be switched over from the operation panel but such that the measurement results can be displayed and in some cases even recorded on the operation panel. In such cases it is expedient if the display already provided on the operation panel also can be used to display the measured yarn tension.
It is expedient to connect the tensiometer to an automatic switch over control device which takes care, e.g. after the representative number of insertion cycles has occurred, that the tensiometer is switched over to the passive position, and which also takes care that the tensiometer is brought in to the respective correct detection position.
In order to minimize the influence of the tensiometer in the detection position for the yarn the tensiometer is structurally combined with the yarn brake, preferably such that the tensiometer uses at least one yarn deflection location of the yarn brake for the measurement.
The tensiometer even may be provided upstream or downstream of a yarn detector, preferably of a weft yarn detector of a weaving machine, and, expediently, even may be structurally combined with the weft yarn detector, preferably such that the tensiometer uses at least one yarn deflection location of the weft yarn detector for the measurement.
Particularly expedient, the tensiometer can be switched into several different detection positions, e.g. depending on the respective yarn quality, which differ from each other e.g. by the respective yarn deflection angle. This is because heavy yarn qualities may need a smaller friction angle for a correct tension measurement than light yarn qualities.
In order to achieve correct measurements despite the different detection positions, it is expedient to provide an electronic measurement evaluation device comprising an automatic compensation circuitry for the different detection positions in order to compensate for the then differing parameters. For the respective detection position at least one position sensor ought to be provided which is connected to the evaluation device. There are namely different force triangles during the measurements in the different detection positions. Those different force triangles would influence the measuring parameters and could falsify the measurements, respectively. The evaluation electronics, however, are able to select the respective correct parameters with the help of the information from the position sensor in order to guarantee correct measurements independent from the respective detection position.
In the case that the yarn processing comprises several yarn channels each of which is supplied by at least one yarn feeding device, expediently one tensiometer is permanently provided in each yarn channel such that it can be switched over, in order to allow the adjustment of the same optimal yarn tension profile for each yarn channel, or even in some cases to adjust an individual optimal yarn tension profile in each yarn channel, respectively.
The invention is applicable to all kinds of weaving machines and knitting machines. A yarn processing system, however, is preferred the textile machine of which is a rapier weaving machine or a projectile weaving machine, although even a jet weaving machine could be provided. In case of knitting machines different machine types could be used like circular knitting machines or flat knitting machines, or the like.